The invention relates to infrared detection technology, and, more particularly, to a high responsivity thermochromic infrared detector and a mode of operation thereof.
Infrared (IR) sensors have been available since the 1940s to detect, measure, and image the thermal radiation emitted by objects. In general, infrared detectors operate by converting IR photons and energy to electrical signals. Detector requirements for missile seekers and forward looking infrared (FLIR) sensors led to high volume production of photoconductive (PC) HgCdTe arrays in the 1970s. However, size and performance limitations of first generation FLIRs required development of self-multiplexed focal plane arrays (FPAs) with on-chip signal processing. Second generation thermal imaging systems thus have used high-density FPAs with relatively few external connections. These second generation thermal imaging devices generally have been fabricated in monolithic and hybrid methodologies. In a monolithic FPA, the detector array and the multiplexing signal processor are integrated in a single substrate. In a hybrid FPA, the constituents, detector arrays and pre-amplifier/multiplexer, are fabricated on separate substrates and interconnected. Further, many detector and readout types are used in two basic FPA architectures of staring and scanning types. The simplest scanning device consists of a linear array in which an image is generated by scanning the scene across the strip. A staring array is the two-dimensional extension of a scanning array; it is self-scanned electronically, can provide enhanced sensitivity, and it is particularly suitable for lightweight cameras and infrared missile seekers.
In any event, FPAs use either photon or thermal detectors. Photon detection is accomplished using intrinsic or extrinsic semiconductors and either photovoltaic, photoconductive or metal insulator semiconductor technologies. Thermal detection relies on capacitive or resistive bolometers. In either case, the detector signal is coupled into a signal multiplexer and read out in video format.
Due to advanced detector materials and microelectronics, large scanning and staring FPAs are now readily available in the short wavelength infrared (SWIR; 1 to 3 .mu.m), medium wavelength infrared (MWIR; about 3 to 5 .mu.m), and long wavelength infrared (LWIR; about 8 to 14 .mu.m) spectral bands. The primary spectral bands for infrared imaging are 3-5 and 8-12 .mu.m, because atmospheric transmission is highest in these bands.
Also, with the more recent advent of inexpensive, uncooled infrared detectors, industrial and law enforcement applications of infrared cameras in particular have dramatically increased. FPA costs are currently relatively similar for all second generation, large format FPA technologies. Therefore, key factors which will determine which FPA technology will most succeed in the future likely will be improvements achieved in availability and cost.
One current vintage of IR detector type encompasses various bolometer detectors, of both resistive and capacitive varieties. Bolometers sense incident radiation via energy absorption and concomitant change in device temperature in both cooled and uncooled schemes. A microbolometer FPA for uncooled applications consists of thin-film semiconductor photoresistors micromachined on a silicon substrate. The uncooled IR FPA is fabricated as an array of microbridges with a thermoresistive element in each microbridge. The resistive microbolometers have high thermal coefficient of resistance (TCR) and low thermal conductance between the absorbing area and the readout circuit which multiplexes the IR signal. As each pixel absorbs IR radiation, the microbridge temperature changes accordingly and the elemental resistance changes. The most recent devices use semiconductor films of about 500 .ANG. thickness having TCR of 2 percent per .degree.C. The spacing between the microbridge and the substrate is selected to maximize the pixel absorption in the 8-14 .mu.m wavelength range. Standard photolithographic techniques are use to pattern the thin film to form detectors for individual pixels.
As the thermoresistive element material, suitable materials include several transition metal oxides and sulfides which exhibit thermochromic behavior, i.e., a reversible phase change from a low temperature semiconductor phase to a high temperature metallic phase. The crystal phase change occurs over a narrow temperature range (i.e., &lt;2.degree. C.) starting at a characteristic transition temperature. The crystal phase change results in a several orders of magnitude change in electrical conductivity in the material. The crystal phase change can be very rapid, occurring within femtoseconds, and is totally reversible. However, there is usually some transition temperature hysteresis, which can be as much 10.degree. C.
A heavily investigated thermochromic material is vanadium oxide (VO.sub.2). Most frequently, its transition temperature is reported to be approximately 68.degree. C. Prior investigators have observed as much as a factor of 10.sup.5 decrease in the bulk resistivity (.omega..cm) of single crystal VO.sub.2 in a &lt;1.degree. C. temperature interval as it transitions from a semiconductor to a metal at approximately 68.degree. C. as illustrated in FIG. 1. See, Kucharczyk, D. et al., J. Appl. Cryst., 12, 370 (1979); Paul, W., Mat. Res. Bull., 5, 691 (1970). Four orders of magnitude change in resistivity have been observed in polycrystalline thin films. See, De Natale, J. F., Mat. Res. Soc. Symp. Proc., 374, 87 (1995); Case, F., Applied Optics, 30, 4119 (1991).
As described in U.S. Pat. No. 5,450,053 to Wood, a microbolometer infrared detector has been developed based on vanadium oxides, preferably VO.sub.2. A resonant cavity is formed between the detector element and the multiplexer beneath it. To accomplish this, a &lt;1000 .ANG. thick vanadium oxide film is deposited on a thin dielectric film (e.g. a Si.sub.3 N.sub.4 film) suspended in air above a multiplexer chip by a dielectric bridge. The microbolometer FPAs are fabricated as an array of microbridges with a thermochromic material element in each microbridge. This provides good thermal isolation from the multiplexer so as not to limit the thermal responsivity of the detectors. The VO.sub.2 in such a microbolometer detector may not have a steep phase transition slope. In fact, U.S. Pat. No. 5,450,053 teaches a preferred operation of the VO.sub.2 detector in the semiconductor phase.
U.S. Pat. No. 5,286,976 to Cole teaches a microstructure design for high IR sensitivity having a two level infrared bolometer microstructure, the lower level having a reflective metal film surface to reflect IR penetrating to that level. The upper and lower levels are separated by an air gap of about 1-2 microns which allows the reflected IR to interfere with the incident IR and increase sensitivity to a higher level. The vanadium oxide bolometer of U.S. Pat. No. 5,286,976 is stated to be preferably operated in the semiconductor phase.
This circumstance of favoring operation of the vanadium oxide in the semiconductor phase in the above-mentioned prior art patents may be attributable, at least in part, to poor stoichiometry, poor crystalline quality (tending toward amorphous morphology), internal stress, or a combination of two or more of these factors which affect the phase transition characteristics. All of these factors are influenced by the method of deposition and post-deposition annealing.
For instance, Curve 2 in FIG. 1 of Begishev, A. R., et al., Sov. Phys. Tech. Phys., 24, (10), 1263 (1979), shows that the phase transition characteristics of oxygen deficient VO.sub.2, i.e., VO.sub.x where x&lt;2, has a relatively small slope, and the transition temperature is shifted to approximately 25.degree. C. Based on this, the relatively low 2%/.degree.C. responsivity of a microbolometer detector device based on the teachings of U.S. Pat. No. 5,450,053 indicates that the phase transition curve has a shallow slope, such as depicted in FIG. 2, or is operated in the semiconductor regime, as indicated as the preferred mode in the U.S. Pat. Nos. 5,286,976 and 5,450,053 themselves, where the TCR is relatively small.
An infrared camera marketed under the name of Sentinel IR camera, has been developed by Amber Engineering, a Raytheon Company, and is based on a 320.times.320 pixel focal plane array reportedly equipped with vanadium oxide microbolometer detectors of the type as taught in U.S. Pat. No. 5,286,976. The Sentinel IR camera produces nearly blemish free IR images (320.times.240 pixels) in the 8-12 .mu.m wave long-wavelength (LWIR) band. The approximately 38.7.times.38.7 .mu.m vanadium oxide detectors are integrated on a silicon CMOS multiplexer chip on 50.times.50 .mu.m centers. Since there are no indium bump-bonds in the FPA, high reliability is obtained, and the FPA is considered to be a monolithic device. The Sentinel IR camera's target detection and recognition range (kilometers) performance, however, may be limited by both resolution of its optics and low thermal responsivity (amps/watt) of the vanadium oxide detectors. A 2% change in responsivity per 1.degree. C. change in detector temperature has been reported for this camera by the manufacturer, which is consistent with the operation of its microbolometer focal plane arrays at approximately 20.degree. C., therefore, most likely in the semiconductor phase.
FIG. 3 depicts an IR detector 300 including a VO.sub.2 thermochromic detector 30, such as taught by U.S. Pat. No. 5,286,976, connected to CMOS multiplexer 31, including an intervening air gap 37, in a manner considered to be used in Amber's Sentinel IR camera. A thin film dielectric member 32 formed of Si.sub.3 N.sub.4 forms a microbridge structure that supports a vanadium oxide microbolometer detector element of approximately 38.7.times.38.7 .mu.m in an array. The detector pitch in Amber's 320.times.240 FPA is 50.times.50 .mu.m, and the fill factor is about 60%. Therefore, the detector dimensions are approximately 38.7.times.38.7 .mu.m. To connect the vanadium oxide film to a measuring circuit, the device circuitry shown includes contacts to pre-amp IC 34a, 34b, an aluminum address line 35, and a polysilicon address line 36. The thickness of the VO.sub.2 film 33 for Amber's Sentinel IR camera device is reportedly 500 .ANG. (500.times.10.sup.-8 cm). The detector is designed to have a nominal resistance of 10 K.omega.. From the detector dimensions and resistance mentioned above, one can calculate a resistivity of 5.times.10.sup.-2 .omega..cm for the vanadium oxide in the microbolometer in the Sentinel IR camera. That is comparable to the resistivity of the vanadium oxide in the phase transition region, as is evident in FIG. 1. Therefore, to achieve the nominal 10 K.omega. resistance necessary for low noise multiplexing, the VO.sub.2 detectors must be 500 .ANG. thick, which is not enough for complete absorption of incident 8-12 .mu.m IR energy in one pass. Either substantially thicker vanadium oxide material or a resonant cavity as taught by U.S. Pat. No. 5,286,976 needs to be used.
Additionally, currently used vanadium oxide based microbolometer detectors include those that are temperature stabilized at about 20.degree. C. by cooling or heating them, as appropriate, with a thermoelectric device on which the FPA is mounted. Since conventional vanadium oxide based microbolometer detectors are preferably operated in a semiconductor phase, as stated in U.S. Pat. Nos. 5,450,053 and 5,286,976, accurate stabilization ostensibly is not required.